Motor graders have a longitudinal main frame which has a dirigible wheel assembly at its forward end, an operator's cab at its rearward end portion, and a traction chassis for the motor and power train behind the cab. The motor grader blade is suspended from the main frame by means of a circle draw bar and a circle. The circle draw bar has its front end connected to the front of the main frame by a ball and socket connection, while the rearward portion of the circle draw bar is suspended from the main frame by hydraulic cylinder and piston means which permit the draw bar to swing in a vertical plane about its front end.
The rearward portion of the circle draw bar constitutes a circle carrying structure, and a circle is supported beneath the carrying structure for rotation about a vertical axis. An internal gear, which is preferably a spur gear because of the relative ease of manufacture, is secured to the circle, and the internal gear and circle are driven by a hydraulic motor through a power train which commonly terminates in a pinion of one sort or another engaging the internal gear.
Two types of prior art circle drives which are known to applicant are disclosed in U.S. Pat. Nos. 2,928,381 and 3,911,758.
There are a number of problems with prior art which drives an internal gear of a motor grader circle from a pinion. Perhaps the most serious is that the forces imposed at the tip of an extended motor grader blade are carried back through the internal gear to the pinion and then to the power input for the spur gear. With a typical motor grader having a maximum extended blade reach of 10 1/2 feet and a circle of 30 inch radius can produce a contact stress in excess of 700,000 psi on the circle drive pinion. This is sufficient to cause metal to extrude away from the contact area and permanently deform the tooth surfaces. Since the only ultimate braking effort is provided by the input worm drive, substantial damage may be sustained by those components.
Another problem in conventional pinion drives for motor grader circles involves the inherent variations in backlash of the pinion to circle mesh. Where the pinion tooth is nearly at the bottom of the tooth's base, there is almost no backlash, while with the pinion rotated approximately half way from the internal gear tooth dedendum to the internal gear tooth addendum the backlash may be about 0.22 inch even before there has been any tooth wear. This permits the induced blade forces to set up vibrations in the circle and the blade mechanism, as well as in the related structure, which substantially reduces the fatigue life of components. The backlash also causes impact loading on the circle drive mesh which further increases the contact stress and produces further damaging forces in the entire circle drive mechanism. Excessive backlash also causes poor surface finish of the graded surface and inaccuracies in grade and slope. This is because backlash can produce blade tip movement. In a motor grader with the dimensions previously stated, a backlash of 0.22 inch produces 0.92 inch movement at the blade tip.
It appears that the special drive pinion of U.S. Pat. No. 3,911,758 probably reduces the backlash problem.
Another severe problem is that a motor grader blade is normally operated at an angle of approximately 45.degree. from the longitudinal center line of the grader, either to the right or to the left. This causes wear to occur only on a very limited number of circle teeth, and only one tooth of the pinion and the circle mesh carries the entire load. This problem is somewhat obviated in U.S. Pat. No. 2,928,381 where there are two circle pinions engaged with the internal gear, so that wear is spread over more than 90.degree. of the internal gear and two teeth are in engagement at all times. Nevertheless, this only alleviates the problem and may complicate backlash problems because of inherent differences in the backlash between the gear meshing at each of two drive pinions with one of the two meshes carrying the load when the blade force moves the circle to take out the backlash.
Further, the use of a pinion as a circle drive requires very accurate adjustment of the shoes which support the circle in order to maintain proper mesh between the drive pinion and the internal gear. The problem of adjusting the circle supporting shoes is complicated by the fact that the entire motor grader circle, grader blade, blade mounting and circle drive is at least coated with dust and may be caked with mud.
The present invention drives the circle internal gear by means of a barrel worm gear which either eliminates or greatly reduces the problems heretofore referred to.
Barrel worm gears employed with an internal gear are disclosed in U.S. Pat. Nos. 2,329,733 and 2,349,642. Hourglass gears, the reverse of a barrel gear, and used with externally toothed gears, are disclosed in such U.S. Pat. Nos. as 3,048,051, 3,386,305 and 3,710,640.